The present invention relates to a method of driving a storage device when a variable resistance element is changed from a high resistance state to a low resistance state (write operation) in the storage device in which a memory cell is composed of a nonvolatile variable resistance element.
An existing storage device, in particular, a storage device using a flash memory has been actively used in recent years, since the electric power for holding storage data is unnecessary. In particular, in a portable terminal device including a cell-phone unit, a flash memory has been widely used as a memory. However, in a storage device using such a flash memory, there is an issue that the data write speed is slow (for example, refer to Non-patent Document 1).
Thus, the applicant has proposed a storage device using a nonvolatile variable resistance element which may have characteristics superior to those of the above-described flash memory (for example, Patent Document 1). This variable resistance element has a stacked structure including a pair of electrodes with a conductive film and an insulating film in between. And, when a voltage is applied between the pair of electrodes so that a current flows from the conductive film to the insulating film, the variable resistance element is changed to the low resistance state, and data is written. On the other hand, when a voltage is applied between the pair of electrodes so that a current reversely flows from the insulating film to the conductive film, the variable resistance element is changed to the high resistance state, and data is erased.
Since the variable resistance element of such a configuration may constitute a memory cell having a simple structure in comparison with the above-described flash memory, there is no size dependency of the element, and a large signal may be obtained. Thus, the variable resistance element has characteristics resistant to scaling. Also, the variable resistance element has advantages that the data write speed by the resistance change may be accelerated to, for example, approximately 5 nanosecond, and, moreover, is operable with a low voltage (for example, approximately 1 V), and a low current (for example, approximately 20 μA).
Non-patent Document 1: Nikkei Electronics, 2002. 11. 18, p.130
Patent Document 1: Japanese Unexamined Patent Publication No. 2005-197634